Solar heating systems for heating water and flowing the heated water to a hot water tank are well known. Water from a tank inside a building is flowed through a supply pipe to a solar collector assembly located outside the building, is heated by solar energy and is flowed through a return pipe back into the building and to the tank. In its simplest form, the solar collector assembly includes a solar collector having an absorber which heats a pipe carrying water from the tank. A circulation pump flows water from the tank to the collector assembly and back to the tank.
A solar heating system of this type can operate even when the outdoor temperature is below freezing. This is because collected solar energy heats the pipe in the collector assembly to a temperature sufficiently high to heat water in the tank. However, the supply and return pipes may be exposed to outdoor temperature below freezing so that water in these pipes freezes, blocks flow and prevents operation of the heating system.
Conventional solar heating systems for use in locations where freezing temperatures occur avoid the problem of frozen supply and return pipes by draining the water from the pipes when solar radiation is absent or insufficient to provide heating. This increases system complexity and prevents use of the system during cold months.
Another solution to the problem of frozen supply and return pipes in solar heating systems is to eliminate water from the pipes and substitute a non-freezing liquid, such as an ethylene glycol-water mixture. The mixture is circulated through the solar collector, is heated by solar energy and is then flowed to a heat exchanger external of the water tank. Heat from the ethylene glycol mixture is flowed to water in the exchanger, which is heated and then flowed to the tank. This solution is less efficient than a system in which tank water is heated directly in the solar collector assembly and is considerably more complicated and expensive than a system using water as a heat transfer medium.
Accordingly, there is a need for an improved solar heating system using water as the heat transfer medium which works when the outdoor temperature falls below freezing. There is also need for a system which automatically clears an ice blockage in a supply or return pipe and resumes operation after an ice blockage has been cleared. The system should have a solar collector using a water heating pipe which is not injured when water in the pipe freezes and expands.